The results of ten clinical trials suggest that supplemental calcium may prevent preeclampsia. Methodologic problems, however, and differences in study design limit the credibility of the results and their generalizability to other patient populations. Moreover, none of the trials has reported the outcome of systematic surveillance for urolithiasis, an important possible complication of treatment. In response to the need for a definitive evaluation of the effects of calcium supplementation, NICHD has conducted a trial at five U.S. university medical centers. Healthy nulliparous patients were randomly assigned to receive either 2 g ingested supplemental calcium daily (n=2250) or placebo (n=2250) in a double-blind study. Medication was administered beginning between 13 and 21 completed weeks of gestation and continued until the termination of pregnancy. Monitoring for the major study endpoints - pregnancy-associated hypertension and proteinuria, preeclampsia, eclampsia, and HELLP syndrome - and for urolithiasis was systematic, standardized, and thorough. It included measurement of blood pressure, proteinuria, and hematuria at uniformly scheduled prenatal clinic visits and surveillance for hypertension and proteinuria during labor, delivery, and the first 24 hours postpartum. CPEP has adequate statistical power to detect a reduction of 43 percent in preeclampsia risk in the calcium group.